The invention relates to a method and arrangement for exhausting gas from a coating material. In the method, the coating material is fed in the bottom part area of a drum rotating around an essentially vertical axis, whereby the rotating motion of the drum causes the coating material to rise up the inner wall of the drum and to discharge from the upper edge of the drum as a thin film against the inner wall of a vacuum tank, wherefrom the coating material flows downwards. The apparatus comprises a tank, including means for providing a vacuum in the tank, a drum arranged inside the tank to rotate around a vertical axis, means for feeding coating material inside the drum, and means for discharging the coating material from the tank.
In the process industry, the mixing of gases, such as air, with the liquids and compounds used in the process typically causes many problems. Particularly in the coating of paper or a corresponding web material, the gas and gas bubbles present in the coating material cause unevenness on the surface of the paper, and even areas completely lacking coating. With some coating materials, the problem is greater than with others, but the problem is emphasized especially with coating materials which bind more gases than others. For example, coating materials containing talcum typically contain a lot of gas due to the properties of talcum.
In addition, the significance of the problem depends on the coating process used. In curtain coating, for instance, the gas content of the coating material may not be higher than 0-0.25 percent by volume. Otherwise the gas bound in the coating may cause uncoated spots in the material to be coated, such as paper or board.
In multilayer curtain coating, the significance of gas exhaustion is increased further. This means that if there are, for instance, three or four coating layers, gas must be exhausted as carefully as possible from the coating used for making each layer.
For exhausting gas mixed with and dissolved in coating material have been developed vacuum deaerators, one known implementation of which is shown in FIG. 1. The device comprises a rotating drum arranged inside a vacuum tank, inside which the coating material is conducted, whereupon due to the centrifugal force, the coating material rises up the inner wall of the drum and discharges from the drum as a thin film colliding with the wall of the vacuum tank.
The problem with prior art vacuum deaerators is their insufficient deaeration efficiency, especially with high-viscosity materials. This is because even under an extremely high vacuum, that is, low absolute pressure, the small air bubbles contained in high-viscosity coating materials are unable to grow large enough to be broken or to be distinguished due to their specific rising speed. Attempts have been made to eliminate this problem by increasing the vacuum, but as a result, the solvent used in the coating material evaporates extremely readily, thus deteriorating the quality of the coating material due to, for example, an increase in the solids content of the coating material following the evaporation of the solvent. As another method are used longer mixing periods, but then the functional capacity of the deaerators remains too low, whereby more devices have to be bought. Moreover, if the separating capacity of known deaerators is increased by increasing the size of the devices, the size of the devices will become excessive and manufacturing costs will increase markedly.